Ultrasonic diagnosis is applied not only to observation of the tissue structure but also to the field (ultrasonic tissue characterization) where physical quantities within the tissue such as the sound speed, the damping coefficient, and so forth, are measured and furthermore, diagnosis images are created based upon the physical quantities thus measured. As a part of the field, the field is known wherein the rigidity of the tissue, i.e., the elastic property is measured. The aforementioned field is being intensely studied since the elastic property of the tissue has close relation to the pathological situation. For example, it is known that the tissue affected by: sclerosing tumors such as mammary cancer, thyroid cancer, and so forth; liver cirrhosis; arterial sclerosis; and so forth, exhibits greater rigidity than the normal tissue. Conventionally, the rigidity of the tissue is detected by touch. However, detection by touch has the disadvantage of difficulty in objective analysis, requires skill of the surgeon, and has the limitations that only the affected tissue having a certain size or more and positioned near the body surface can be detected.
On the other hand, a method is known wherein static pressure is applied to the body surface so as to compress and deform the tissue, and the strain within the tissue corresponding to the applied pressure is measured using ultrasonic wave in order to estimate the elastic property of the tissue (J. Ophir, I. Cespedes, H. Ponnekanati, Y. Yazdi, and X. Li, “Elastography: A quantitative method for imaging the elasticity of biological tissue”, Ultrasonic Imaging, Vol. 13, pp. 111-134, 1991). The conventional technique has been developed based upon the fact that the tissue having great rigidity exhibits small strain thereof under pressure, and on the other hand, the tissue having small rigidity exhibits great strain thereof under pressure. That is to say, with the aforementioned conventional method, static pressure is applied to the tissue, and the elastic property of the tissue is estimated based upon the strain distribution within the tissue under pressure thus applied.
Specifically, normal measurement of ultrasonic echo signals (RF signals without application of pressure) is made using an ultrasonic diagnosis apparatus with an ultrasonic probe without pressure applied to the tissue through the ultrasonic probe. Subsequently, the surgeon applies pressure to the tissue through the ultrasonic probe to a slight degree (around several percent), following which the ultrasonic echo signals (RF signals under pressure) passing through the tissue to which pressure is applied are measured. Then, the displacement distribution which represents displacement of each point of the tissue due to the pressure thus applied is estimated based upon the RF signals with and without application of pressure of the tissue using the spatial correlation method.
The spatial correlation method has a mechanism wherein the displacement distribution within the tissue under the applied pressure is estimated based upon the RF signals (or envelope signals of the RF signals) with and without application of pressure applied to the tissue by template matching using a two-dimensional correlation function. That is to say, a two-dimensional correlation window (template) having a certain size is applied to the RF signal data corresponding to the tomographic data without pressure applied to the tissue so as to estimate displacement of a desired measurement point on the two-dimensional surface by detecting the maximum correlation between the RF signal data to which the correlation window has been applied and the RF signal data under pressure applied to the tissue using the autocorrelation processing. The aforementioned autocorrelation processing is performed for each measurement point set in the shape of a grid, for example, whereby the strain distribution is estimated. In general, the processing using the spatial correlation method is performed with poorer precision of displacement detection in the horizontal direction (scanning direction of the ultrasonic beam) than in the axial direction due to rougher sampling in the horizontal direction than in the axial direction. As described above, the spatial correlation method has the advantage of enabling the user to estimate the two-dimensional displacement vector. Furthermore, while the aforementioned spatial correlation method has the disadvantage of precision of the estimated displacement being limited by the sampling pitch, the spatial correlation method has the advantage of enabling the user to estimate the displacement distribution even in a case wherein the tissue is greatly deformed (e.g., around 5%). However, the spatial correlation method has the disadvantage of being calculation-intensive for the spatial correlation processing, leading to difficulty in processing in real time, unlike the conventional ultrasonic diagnosis.
Accordingly, it is an object of the present invention to provide a method for obtaining the displacement distribution, strain distribution, and elastic modulus distribution, in real time.